The present invention relates to a method and apparatus for producing a magnetic recording medium. Particularly, it relates to a method and apparatus for producing a magnetic recording medium having a high coercive force.
In recent years, coupled with the progress in information treating techniques including computers, the level of requirements for high density recording has increased with respect to magnetic recording media such as magnetic disks used for external memory devices thereof.
At present, it is common to employ a thin cobalt alloy layer formed epitaxially on a thin chromium undercoating layer by sputtering, as a magnetic layer for a magnetic recording medium used as a longitudinal recording magnetic disk. With this thin cobalt alloy magnetic layer, it is necessary to impart a high coercive force as a magnetic property required for the high density recording. A number of reports have been made on such property (e.g. "New longitudinal recording media Co.sub.x Ni.sub.y Cr.sub.z from high rate static magnetron sputtering system", IEEE Trans. Magn. , Mag-22, No. 5, (1986), 334; Japanese Unexamined Patent Publication No. 79233/1988; Japanese Unexamined Patent Publication No. 79968/1988).
As stated in such reports, the coercive force of a thin cobalt alloy magnetic layer increases with the thickness of the chromium undercoating layer. However, when the thickness exceeds a certain upper limit value, the coercive force shows a saturation characteristic, and it becomes difficult to further increase the coercive force. For example, Japanese Unexamined Patent Publication No. 79968/1988 teaches that when the thickness of a thin chromium undercoating layer exceeds 1,500 .ANG., a saturation tendency is observed such that the coercive force of the magnetic layer no longer increases, and when the thickness of the undercoating layer is less than 1,500 .ANG., the coercive force tends to be remarkably low at a practically useless level.
Further, the coercive force increases with a reduction of the thickness of the thin cobalt alloy layer. However, the reduction of the thickness of the cobalt alloy layer leads to a decrease in the production output, and it is difficult to practically reduce the thickness beyond a certain level. Furthermore, the coercive force may be increased to some extent by selecting the sputtering conditions such as the pressure of the gas for forming or the substrate temperature during the formation of the magnetic layer, but such improvement is limited.
It is also known to improve the coercive force of the magnetic layer by conducting sputtering under such a condition that a negative bias voltage is applied to the substrate (The 35th Lecture Meeting of Applied Physics Association, Spring 1988, 29a-c-9,10). However, in a continuous layer-forming apparatus wherein the substrate moves, it is necessary to apply a negative potential to the moving substrate, and as a result the apparatus is complicated. Besides, it is necessary to apply the negative potential over a wide area i.e. not only in the vicinity of the target. Therefore, it is possible that an abnormal electric discharge will take place, which in turn leads to damage to the layer-forming apparatus. Thus, this method has various problems as an industrial layer-forming method.